Can Genetic Engineering Save the Florida Orange?

Researchers have found that thermotherapy, or baking infected citrus trees with solar radiation to 100 degrees Fahrenheit for a few days, kills some of the citrus greening bacteria and allows the tree to survive a few more years.

Photograph by Craig Cutler/National Geographic

PUBLISHED September 13, 2014

Citrus greening, the plague that could wipe out Florida's $9 billion orange industry, begins with the touch of a jumpy brown bug on a sun-kissed leaf.

From there, the bacterial disease incubates in the tree's roots, then moves back up the trunk in full force, causing nutrient flows to seize up. Leaves turn yellow, and the oranges, deprived of sugars from the leaves, remain green, sour, and hard. Many fall before harvest, brown necrotic flesh ringing failed stems.

For the past decade, Florida's oranges have been literally starving.

Since it first appeared in 2005, citrus greening, also known by its Chinese name, huanglongbing, has swept across Florida's groves like a flood. With no hills to block it, the Asian citrus psyllid—the invasive aphid relative that carries the disease—has infected nearly every orchard in the state.

By one estimate, 80 percent of Florida's citrus trees are infected and declining.

The disease has spread beyond Florida to nearly every orange-growing region in the United States. Despite many generations of breeding by humanity, no citrus plant resists greening; it afflicts lemons, grapefruits, and other citrus species as well. Once a tree is infected, it will die. (See "Can Parasitic Wasps Help Save America's Citrus?")

Yet in a few select Floridian orchards, there are now trees that, thanks to innovative technology, can fight the greening tide. These trees have the potential to keep Florida orange juice on your breakfast table—provided you are willing to drink the juice of oranges that have been genetically modified to contain genes from spinach. (Read "The Next Green Revolution" in National Geographic magazine.)

The trees are the work of Erik Mirkov, a plant pathologist at Texas A&M University who has spent his career applying the tools of biotechnology to citrus. Over the past few years, his research on genetically modified oranges has gone from an academic sideshow to one of the great hopes of the industry.

It's highly unlikely, researchers and growers agree, that oranges will remain in Florida unless new, modified strains like Mirkov's are widely grown—a view endorsed by the National Research Council several years ago.

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Citrus greening incubates in the tree's roots, making it difficult to detect infection. A healthy citrus root system is shown at left, and an infected one at right.

Photograph by Craig Cutler, National Geographic

Orange Juice Jitters

The pressure to find solutions keeps growing. Even without disease, the orange industry is under stress. It's losing land to housing developments; it's losing customers to the spreading notion that orange juice is a sugary, not healthy, drink.

This past year, Florida produced only 104.4 million boxes of oranges, the lowest total in nearly three decades; this next season could be even worse. There's rampant fear that Florida orange groves are a couple of years away from full collapse.

"Growers are calling all the time because they're watching their livelihood collapse in front of them," says Robert Shatters, a molecular biologist at the U.S. Horticultural Research Lab in Fort Pierce, Florida. "We feel that pressure very strongly. We realize time is ticking."

The citrus industry, slow to prevent the greening disease, has partially redirected its advertising budget and invested heavily in research—reportedly $90 million so far. Southern Gardens Citrus, one of the largest growers, supports Mirkov's work. The federal government, too, has contributed, with this year's farm bill directing $125 million toward the fight against citrus greening.

Growers and scientists desperate for ways to sustain existing trees have already adopted temporary measures—targeted applications of antibiotic, for instance, or of fertilizer and water to reduced roots.

Researchers have found that heating trees inside plastic tents can prolong life spans by killing some of the bacteria; in California, researchers are releasing parasitic wasps from Pakistan that attack Asian citrus psyllids.

And close to commercialization—so close that Shatters, the scientist behind them, can't talk specifics anymore—are chemical tree coatings that target the specific biology of the psyllid.

More ambitious projects include efforts to replace the Asian citrus psyllids with ones that have been rendered incapable of spreading the disease, much like recent work that has combated malaria-carrying mosquitoes.

And inspired by trials in humans that have used modified HIV to attack cancer, citrus scientists have engineered a common citrus virus to carry molecules known to attack Candidatus liberibacter asiaticus, the bacterium that causes greening—with limited success so far.

For citrus researchers, this past decade has brought mixed emotions. The objects of their study are in trouble, yet their work has never been more important. "It was the best of times, it was the worst of times," says Shatters. "As a scientist, it's been the most exciting time in my life."

That's especially true for Mirkov. For much of his early career, he couldn't interest anyone in growing his biotech citrus. Now, he gets unsolicited calls from Florida all the time. "All my beautiful citrus trees are dead," they say. "When can I get your trees?"

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The Asian citrus psyllid feeds on citrus trees and carries the disease from tree to tree. In Florida, most citrus trees are infected.

Photograph by Sam Droege, USGS Patuxent Wildlife Research Center

Birth of a New Orange

Back in the 1990s, as young scientist who had just earned his doctorate, Mirkov became fascinated with applying the nascent tools of biotechnology to citrus. The greatest threat to the fruit then was a virus called tristeza, a Portuguese word for sadness, after the sadness that stems from its arrival. The virus had spread to Florida—"Florida gets everything first," Mirkov says, because it's a global crossroads with a hospitable climate—and he feared its arrival in Texas.

Other researchers had created papayas that resisted ringspot virus by inserting a small bit of the virus's DNA into the plant's genetic code. Using a similar strategy, Mirkov created Ruby Red grapefruit that resisted tristeza. With permission from the U.S. Department of Agriculture, in 2000, he planted the country's first biotech citrus trees in an orchard near his lab.

They're still there, 14 years later; Mirkov periodically renews his research permit, and each year he must pick and grind the fruit and plow it into the soil, because selling the fruit or planting the trees outside a research plot is not allowed. No company has sought to commercialize the trees, which would require a lengthy and costly deregulation process. It wasn't worth the hassle.

In 2000, the citrus world had a different priority: canker, a bacterial disease that had reemerged and run wild in Florida. Mirkov began exploring how citrus plants could resist bacteria. His virus resistance tricks wouldn't work, so he looked at genes that produce antimicrobial proteins.

There were many intriguing sources: scorpion and honeybee venom, sarcophagus beetle toxin. They worked well, but then he imagined what would happen if the public found out their orange juice had a bit of sarcophagus beetle in it. "If I'm going to do this," he decided, "it's got to be with genes that were commonly consumed by everyone."

Mirkov was fortunate that a group of Spanish scientists had dedicated themselves to grinding up a wide variety of plants to discover their defensive proteins. They had identified a potent one in spinach, of all things, that attacked a wide variety of bacteria and fungi. Spinach has several such proteins, it turned out; incorporating just a couple into a tree might give it resistance to a broad spectrum of diseases.

That's exactly what Mirkov and his colleagues did, copying the genes that encode several of these proteins into an orange tree's DNA.

At first they targeted canker bacteria. But once greening appeared, it was obvious to Mirkov that they might rework the system to deliver the antimicrobial proteins to the innermost layer of bark—where the greening bacteria disrupt the flow of the tree's nutrients. By 2007, he had begun working with Southern Gardens.

Fields of Green

Before field trials, Mirkov first tests his biotech trees in his "psyllid house," an insect-proof greenhouse that is creeping with the jumpy brown bugs. It's a much more severe environment than the trees would see in real life, yet by the second and third generations, his greening-resistant orange trees continued to thrive even after 16 months in the greenhouse.

"The plants were literally covered in psyllids, and there was no infection at all," Mirkov says. "Those are the lines that go into field trials."

Those field trials have also met with success: Some of his first-generation trees are resisting the disease to some degree, even after five years; a much larger trial of second- and third-generation trees is going well after almost two years.

Overall, Mirkov is on his fifth version of the technology, and they've begun applying it to the whole diversity of Florida citrus: grapefruit, lemon, and, importantly, the rootstocks, like sour orange, that growers use as a base for their trees.

Southern Gardens is now seeking to deregulate these oranges for free use, a long process that requires approval from the Environmental Protection Agency, the Department of Agriculture, and the Food and Drug Administration. It's a process that tends to dissuade academic scientists; except in a few rare cases, like genetically modified papaya in Hawaii, only wealthy seed companies pursue deregulation of biotech crops.

Other researchers, like the University of Florida's Jude Grosser, are pursuing biotech oranges, but Mirkov's are the closest to market, experts say.

So just how long will it take? Barring a long regulatory holdup—far from a sure bet—the first commercial planting should come in three to four years, says Mirkov.

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These tents hold host plants that Asian citrus psyllids being raised for research feed on to adulthood.

Photo by Craig Cutler, National Geographic

GMO O.J. OK?

One great remaining question is whether consumers will drink juice from genetically modified oranges. It's a dilemma that Southern Gardens has been worried about for years, as Amy Harmon of the New York Timesdocumented in a long feature last year.

A raft of market research has probed this question, Shatters says, and found that, unsurprisingly, the public is more open to biotech orange juice when it's presented as the alternative to no high-quality orange juice at all.

While GMO crops are grown on nearly half of the United States' farmland, and they supply cornmeal, oils, and sugars that tens of millions of Americans eat daily, there has never been a case quite like orange juice.

When Mirkov read the comments on the New York Times story, he was heartened to see how many people said they might give GMO O.J. a shot. The negative comments focused heavily on Monsanto—even though the company is not involved in the orange work. "They just have this preconceived idea of big bad Monsanto doing this again," Mirkov says.

Until the fruit is out there, it's hard to say whether consumers will buy it. The idea of spinach DNA in an orange, even if safe and odorless, could just be too much, Mirkov worries: "Some people might say, 'I guess I'll drink apple juice instead.'"

Meanwhile, given the slow pace needed to develop biotech oranges, any method that can buy time for existing trees is welcome. One of the most promising involves Mirkov's old friend, citrus tristeza.

Just as "disarmed" viruses like HIV can make effective drug-delivery devices, citrus pathologist Bill Dawson of the University of Florida and his team have shown that tristeza, which spreads to every part of a tree, can deliver treatments like Mirkov's spinach proteins. Since the genes for the proteins would be inserted into the virus, not the tree, the oranges themselves would not be genetically modified. And since the virus can be grafted into existing trees, the system might save some infected citrus trees, or at least extend their lives. Every year counts.

In the end, it's unlikely even biotech trees, if they happen, will eradicate the problem; complete immunity is unlikely, and the bacteria may evolve ways of overcoming the newly engineered defenses.

In our hyperconnected world, Florida was living on borrowed time before greening arrived. If the industry survives, the disease will always have to be managed, says Jim Graham, a soil microbiologist at the University of Florida.

"This disease will always be the most devastating and difficult of citrus that we know and will ever know," he says. "We're stuck with a very difficult problem from here on in."